Hemicellulose represents a relative large fraction in wood materials, being typically from 20 to 40 weight % of the wood material depending on wood species. In chemical pulping processes, such as Kraft pulp and dissolving pulp processes, wood is delignified. In addition to lignin, hemicellulose is separated to a large extent from cellulose in the chemical pulping process, and the final pulp product contains only small amounts or no hemicellulose, depending on the process. Therefore, approximately half of the wood is dissolved in a chemical pulping process. In the pulping processes, such as Kraft pulping or sulphite pulping (dissolving pulp), the hemicellulose fraction that is separated from the pulp is mainly combusted and not valorized.
In the main pulping process, Kraft pulping (sulphate pulping or cooking), wood lignin and partly hemicellulose are dissolved in severe conditions in a digester (cooking). The hemicellulose degradation products in the resulting liquor (black liquor), are quite complex and their separation and purification from the liquor is difficult and its suitability for microbial processes is poor. For that reason the black liquor is combusted in the recovery boiler to produce electricity and thermal energy. However, hemicellulose has significantly lower heating value than lignin, the combustion of dissolved hemicelluloses does not constitute optimal economical use of this resource. Therefore, hemicellulose should preferably be extracted, prior to cooking, to produce higher value products. One option to valorize hemicellulose is to convert it into chemicals and/or to biofuels (e.g. ethanol, butanol) by microbiological processes. However, this hemicellulose pre-extraction before pulping should not decrease pulp yield or quality.
Dissolving pulp (also called dissolving cellulose) is a bleached wood pulp that has a high cellulose content (>90%). Dissolving pulp is a chemical pulp grade used as the raw material for a wide variety of cellulose derivatives, for example rayon (textiles), cellophane, cellulose acetate and methylcellulose. Dissolving pulp is made by the sulphite process or the kraft process with a step to extract the hemicelluloses before the pulping process. Hemicellulose is a byproduct which is largely being wasted at mills today as it is often combusted in the boilers and most of the energy is lost as waste heat.
Several different hemicellulose pre-extraction methods prior to Kraft-pulping can be applied, such as pressurized hot water extraction (hydrothermal treatment) and various organosolv methods using alcohols (e.g. ethanol, methanol) and/or organic acids (e.g. acetic acid, formic acid, malic acid, lactic acid or peracetic acid) possibly supplemented with acid catalyst such as SO2. The methods can facilitate removal of mainly hemicellulose or both lignin and hemicellulose from pulp fraction. In majority of these pre-extraction methods, hemicellulose is obtained at least partly in oligomeric form. The oligomers need to be hydrolysed to monomers before they can be converted to end products by microorganisms. The conversion of sugar oligomers in hemicellulose to sugar monomers can be performed by specific enzymes, such as xylanases, arabinases, galactosidases and/or mannananes, depending on the type of hemicellulose (depending on the wood species). Such enzymes are produced by certain microorganisms, typically fungi or bacteria.
A biorefinery concept which includes pre-extraction of hemicellulose before pulping process and utilization of hemicellulose fraction to ethanol or chemicals production has been suggested in the prior art. A review of hemicellulose pre-extraction methods is presented in Huang et al. (2008) and an overview of Kraft-pulp biorefinery including hemicellulose pre-extraction by Marinova et al. (2009). Patent publication US20090165968 describes a method to pre-extract hemicellulose before Kraft-pulping process, which enhances the susceptibility of hemicellulose to biological fermentations for ethanol and chemicals. Production of ethanol from pre-extracted hemicellulose from eucalyptus Kraft-pulping process is described in Mendes et al. (2010).
Although in the prior art has been described various methods by which wood material and in particular hemicellulose can be extracted for pulp industry processes, there is still a need for new processes which would facilitate the fractionation and further valorization of wood material, in particular hemicellulose, for the pulp industry processes.
The degradation and utilization of lignocellulosic material is important also on other industrial fields than in pulp industry processes. Single-cell oils have traditionally been used as special products e.g. in health foods. Similar kind of production process has also been described for the production of lipids for biodiesel production. However, as the product is an inexpensive commodity chemical, the process costs should not be on the level of the process costs of special products. Further, the lipid yield by heterotrophic microorganisms is typically very low, less than 20% weight percent of the fed sugar, in the best case 22-24% from available sugars can be transformed to oil. Due to these reasons, the utilization of low-cost raw materials, such as lignocellulosic feedstock, for oil production is necessary.
A single cell oil production process using microorganisms generally comprises cultivating microorganisms in aerated bioreactors, allowing cells to accumulate lipids, harvesting lipid-rich cells and recovering oil from cells.
Utilization of cellulosic feedstock for oil production has been suggested in some recent patent publications. US 2009/0064567 A1 discloses the production of biological oils by heterotrophic fermentation by growing microorganism of the kingdom Stramenopile by using cellulose-containing feedstock as a main source of carbon. The cellulose is hydrolyzed by enzymes or by other microorganisms capable of saccharifying cellulose. WO2009/0011480 A1 discloses the production of biological oils from depolymerised cellulosic material by microalgae and fungi.
Patent publication US2009/217569 discloses single cell oil production from various lignocellulosic and other material hydrolysates, including wood and pulp and paper industry residues for manufacturing biofuel. The method comprises treating source material with water, acid or alkali and contacting filtrate or precipitate with lipid-producing microorganism. The prior art describes also lipid production directly from polymeric sugars in lignocellulose, such as xylan by Fall et al. (1984), or cellulose by Lin Hui et al. (2010) by organisms capable of producing cellulases or hemicellulases.
The enzymatic hydrolysis is typically performed in a separate step from biofuel production process by commercial enzymes. Typically commercial enzymes are bought, and produced outside the actual biofuel production process. The price of enzymes is the major cost factor in the conversion of cellulosic materials to biofuels by microbiological processes.